How to calculate nitrogen use efficiency on-farm
© Tim Scrivener Nitrogen use efficiency (NUE) is used to demonstrate the relationship between a soil’s total nitrogen input compared with its nitrogen output.
Calculating NUE will give growers a starting point from which they can improve how nitrogen is used – particularly timely given the fertiliser price hike.
Adopting a complete nutrition strategy that focuses on soil health and crop yield potential can improve profitability, resource efficiency and sustainability on farm.
Generally, a typical mineral soil recovers 60% of the nitrogen it receives. This rate increases on sandy soils, but reduces on calcareous soil types.
Experts advise farmers to aim for 70-80% NUE, which means there is considerable scope for improvement.
We take a look at how growers can calculate their nitrogen use on-farm, and discuss a number of ways this recovery rate can be improved.
See also: AHDB guidance to find optimal fertiliser rate as prices soar
Finding the optimum nitrogen use efficiency
When it comes to finding the optimum nitrogen use efficiency (NUE), a rate of 100% is, surprisingly, not the desired outcome.
Instead, growers should aim for 70-80% as an optimal operating space, advises Allison Grundy.
A NUE value below this – particularly below 50% – suggests a nitrogen surplus. This increases the risk of nitrogen leaching, which comes with both an economic and environmental cost.
On the other hand, too high NUE values (more than 90%) indicates mining of soil nitrogen stocks, which sees depletion of the soil mineral nitrogen supply.
This can be detrimental to the health of the soil, particularly the biology, in the long term.
“Growers should not aim for a nitrogen use efficiency of more than 100%, otherwise this will effectively begin risk the soil-mining process.
There is a certain amount of nitrogen required within the soil profile, which all the soil biology, macro- and micro-fauna rely on to carry out essential soil functions,” she says.
The general consensus is that there is much room for improvement in nitrogen management to reach the desired 70-80% NUE.
Calculating NUE
In its most basic form, nitrogen use efficiency is calculated by dividing nitrogen output by nitrogen input.
The overall efficiency of nitrogen depends on the system and its management.
Independent crop adviser Allison Grundy explains: “NUE is low when the nitrogen output in harvested products is low and the nitrogen input is high.
“This efficiency increases as the nitrogen output in the harvested products increases and/or the nitrogen input decreases.”
To measure nitrogen use, the following are required:
- Harvested grain yield (t/ha or kg/ha)
- Total crop nitrogen supply:
a. Soil mineral nitrogen (plus estimate of soil nitrogen mineralisation)
b. Organic nitrogen applications
c. Inorganic fertiliser applications. - Harvested grain N percentage / protein content
Good data capture is absolutely vital. It is important a whole-crop or grain sample is taken at harvest and sent off for total N and dry matter analysis. The yield data must also be corrected to the same moisture content.
A worked example
Nitrogen output
- Grain yield at 100% dry matter (DM) = 10t/ha
- Grain protein at 100% DM = 13.5%
- Grain nitrogen = 13.5 / 5.7 = 2.37% N
Total nitrogen output = grain yield x grain N percentage = 10,000 x 2.37% = 237kg N/ha
Nitrogen input
- Soil nitrogen supply before any nitrogen applications = 80kg N/ha (soil mineral nitrogen plus estimate of soil nitrogen mineralisation plus spring crop nitrogen)
- Organic manure N application = 40kg N/ha
- Inorganic nitrogen applications = 220kg N/ha
Total nitrogen supply = 80+40+220 = 340kg N/ha
Nitrogen use efficiency = nitrogen output / nitrogen input = 237 / 340 = 70%
Fertiliser recovery efficiency
Another key measurement growers may be interested in is the fertiliser recovery efficiency calculation. This aims to find the percentage of applied nitrogen fertiliser that is successfully recovered by a crop.
Fertiliser recovery (%) = (crop N uptake + N applied) – (crop N uptake + 0 N applied)/N applied x 100
In order for growers to carry out this calculations, an area of unfertilised crop in the same field is required. Again, a whole crop or grain sample must be sampled at harvest and the grain analysed for total N and dry matter.
If a whole-crop sample is taken, the N in both the straw and grain must be measured, and the weight of the crop over a known sampling area recorded. This is so you can calculate the kg N/ha, says Ms Grundy.
She also notes that nitrogen fertiliser use efficiency varies with soil type.
Improving nitrogen use efficiency on farm
Growers will certainly be focusing their minds on how to improve nitrogen use efficiency this season, given the current fertiliser price hike.
Allison Grundy advises starting with the soil: “It is vital that we look after our natural resources by identifying what our soils can deliver for us, as well as properly valuing the importance of organic manures.”
Start with a baseline test of soil pH and other macronutrients, as these basic principles are absolutely critical to the effective use of nitrogen.
“Work on the most recent soil analysis, as the balance of other nutrients and the soil pH have a huge effect on the crop’s ability to recover and use nitrogen,” she says.
Then identify the correct crop nitrogen requirement and, importantly, estimate the optimal yield potential of the crop.
Make the most of the RB209 guidelines and select the correct rainfall table and soil texture to identify the appropriate soil nitrogen supply index (SNS).
Understanding how much soil N supply there is determines the net nitrogen requirement.
Analyse the nutrient value of any compost, digestate and livestock manures, as nitrogen supplied from these materials needs to be accounted for.
You must accurately estimate the crop yield potential, as overestimating crop yields can result in excessive amounts of nitrogen being applied.
Again refer to the correct RB209 SNS tables by considering soil type, previous cropping and rainfall. Refer to historic yield maps to gauge the yield potential of fields.
Only then should growers consider the application of nitrogen fertiliser.
A little-and-often approach is associated with improved nitrogen use efficiency, rather than a smaller number of larger applications.
Measuring the soil nitrogen supply in spring before fertiliser nitrogen has been applied can be very beneficial where soil nitrogen levels are predicted to be large or unknown.
“Getting the basics right before engaging in more advanced technologies such as remote sensing and satellite imagery is the best way of realising the gains those technologies are capable of delivering,” concludes Ms Grundy.
